TRANSPIRATION 



39 



C=^ 



altered by them, and this in turn affects the amount of transpiration. Among 

 physical influences the dampness of the air holds a foremost place ; its effects 

 are so obvious that further explanation is unnecessary. Similarly with tem- 

 perature ; every increase in temperature must cause an increase in transpiration 

 — every decrease must retard it. If the plant be at a higher temperature than 

 the environment it can still give off water vapour into the air even though the 

 latter be saturated ; this higher temperature is attained by respiration or by the 

 absorption of light and heat rays, often aided by the presence especially of colour- 

 ing matters (Stahl, 1896). Transpiration is also increased by oscillation, for the 

 plant is thus taken out of the saturated atmosphere produced over its surface in 

 consequence of transpiration, and brought into a new region not yet saturated. 

 The same effect is produced by moving the air rather than the plant, and hence 

 every breath of wind aids transpiration. The dampness of the soil has an influence 

 which is somewhat less obvious. Dry soil hinders transpiration because it 

 retards absorption of water ; owing to the lack of a reserve of water the cell-sap 

 becomes more concentrated in the transpiring organs, and hence is less ready to 

 give off water vapour into the air. Concentrated salt-solutions, if the root has 

 to absorb water from them, 

 act in the same way as a 

 dry soil, although dilute 

 solutions also have an in- 

 fluence on transpiration 

 which has not as yet been 

 fully explained. Dilute 

 acids retard, and dilute 

 alkahs accelerate, transpira- 

 tion. Probably in these 

 cases the explanation is not 

 purely physical, but must 

 be sought for in alterations 

 in the characters of the 

 plant itself, having their 

 cause especially in the acti- 

 vities of the guard-cells of 

 the stomata. 



We have not as yet discussed this question of the activity of the guard cells, 

 and so far, our treatment of the subject would suggest that the stomatal aper- 

 ture was always of the same size. That is, however, by no means the case. 

 On the contrary, the guard-cells are capable of opening and closing the stomatal 

 slit according to conditions, and thus of allowing of the most varying amounts 

 of transpiration, from nothing upwards. Variations in the size of the pore are 

 attained by a very simple method, viz. by varying the degree of curvature of 

 the guard-cells. To understand the mechanism of the process it will be 

 necessary for us to study the structure of the stoma somewhat more in detail 

 than we have already done. We may select for detailed study the stoma of 

 Amaryllis, whose structure has been elucidated most thoroughly by Schwen- 

 DENER (1881). Other plants exhibit other adaptations than those seen in 

 Amaryllis, but the mechanical principles involved are fundamentally the same for 

 all (compare Haberlandt, 1896, and Copeland, 1902). Fig. 8 shows a stoma 

 of Amaryllis, both in the open and in the closed condition, in surface view 

 and in transverse section. The latter (Fig. 8, 7) shows the asymmetrical form 

 of the guard-cells in relation to the line S, which separates the concave from 

 the convex side. While the convex half forms almost exactly a half circle, the 

 outer contour of the concave side is much more complicated, and as a conse- 

 quence the intercellular space between the concave edges of the guard-cells also 



Fig. 8. /-IV, stomata a{ Amaryllis formosissima (after ScHWEN- 

 DENER). /, in section; 77, surface view ofhalf-stoma; ///, surface view 

 of a closed, and IV, of an open stoma ; F, VI, for explanation see text. 



